1. Field of the Invention
This invention relates to apparatus for producing thermoplastics mouldings.
2. Prior Art
It is known to produce structural foam plastics mouldings in a horizontally acting injection moulding machine. Commonly pellets of plastics material and a blowing agent are heated in a screw conveyor and fed to a nozzle for introduction into a mould space defined by a moulding tool mounted between a fixed platen and a movable platen operated by a hydraulic ram. Normally the viscous material is introduced through an axial passageway in the fixed platen. Within the passageway is formed a sprue which is integral with the moulding and which must subsequently be removed from the moulding in a finishing process before the moulding is painted. This finishing process necessitates the sprue being cut below the surface of the moulding leaving a recess which has to be filled and sanded. The finishing process is thus expensive and, in practice, cannot be performed immediately after moulding because of the tendency for the blowing agent to remain active for a considerable period of time. This intervening period can thus lead to a high loss rate of mouldings due to damage.
Known arrangements introduce the plastics material around or past a retractable core member which is then moved to close the passageway. In this case no sprue is formed which requires subsequent cutting. On the other hand, other problems are created since the material continually surrounding the core member has to be kept viscous and at the end of each moulding cycle there is created a solidified length of material which the core member thrusts into the mould space during the next cycle and which may spoil the resultant moulding.
The same applies both during the moulding of plastics material without a blowing agent, or alternatively the production of mouldings which require the injection of a fluid into the plastics moulding to create a pressurised mass therein which urges the plastics material into contact with the mould surfaces.
An object of the invention is thus to provide a moulding cycle which involves the formation of a sprue but which at least reduces the post-moulding treatment of the resultant moulding.